There are various standards and systems in wireless communications at present, and pluralities of frequency bands (multi-band) in pluralities of systems (multi-mode) are required for mobile communications equipments such as mobile phones, etc., for the convenience of increased numbers of users. There are various access systems in mobile phones available worldwide at present, and pluralities of access systems are used in some areas. Since a first-generation, analog, mobile phone system utilizing FDMA (frequency division multiple access), there have been PDC (personal digital cellular) as a second-generation digital mobile phone system using TDMA (time division multiple access), GSM (global system for mobile communications) and DCS (digital cellular system) mainly used in Europe, and DAMPS (digital advanced mobile phone service) and PCS (personal communications service) mainly used in the U.S. as presently main multiple systems. GSM, DAMPS, DCS and PCS are sometimes called GSM850, GSM900, GSM1800 and GSM1900, respectively. In the GSM system, data-transmitting technologies such as GPRS (general packet radio service) and EDGE (enhanced data GSM environment) are used with its mobile phone network.
A CDMA (code division multiple access) system has recently been spreading in the U.S., Europe, China, Korea and Japan. Because the CDMA system is better than the TDMA system in capacity for accepting many users, the CDMA system is a technology attracting the most attention at present. Because technologies for achieving high control of transmitting power were established, they have been being applied to mobile communications systems with drastically changeable length and loss of wireless transmission paths.
The CDMA system according to the IMT-2000 standard determined by the International Transmission Union (ITU) is used for a so-called third-generation mobile communications system providing various services, etc. utilizing high-speed data transmission and multi-media.
As the CDMA system, there are CDMA 2000, a higher standard of cdmaOne, standardized by IS-95 (interim standard-95), which is the U.S. standard; DS-CDMA (direct spread code division multiple access), which is also called W-CDMA, and TD-CDMA (time division code division multiple access) as the universal mobile telecommunications systems (UMTS) of the European standard; and TD-SCDMA (time division synchronous code division multiple access), which is a Chinese standard. TD-CDMA and TD-SCDMA are different from other CDMA systems in that they use a time division duplex (TDD) technology of switching the uplink and downlink transmissions in a short period of time for communications.
Because a law imposing telecommunications carriers to identify transmitting positions at the time of emergency call by mobile phones has been enforced in the U.S., the navigation function of a global positioning system (GPS) has recently been added to mobile communications apparatuses such as mobile phones, etc. In Japan, etc. too, mobile phones with GPS navigation functions have been increasing for the convenience of users.
In data-transmitting technologies, there is WLAN, which has pluralities of standards, such as Bluetooth (R) using an FHSS (frequency hopping spread spectrum) system in a 2.4-GHz ISM (industrial, scientific and medical) band; IEEE802.11a using an OFDM (orthogonal frequency division multiples) modulation system in a 5-GHz band; IEEE802.11b using a DSSS (direct sequence spread spectrum) system in a 2.4-GHz ISM (industrial, scientific and medical) band like Bluetooth(R); IEEE802.11g using an OFDM (orthogonal frequency division multiples) modulation system in a 2.4-GHz band, etc. Mobile phones using these data transmission technologies are also increasing.
It is required that high-frequency devices in front-end parts (high-frequency circuit parts) of such mobile communications apparatuses can be used in pluralities of systems in pluralities of frequency bands, with reduced size.
JP2000-156651A discloses a communications terminal capable of being used in both a TDMA system having different transmitting/receiving timing and a DS-CDMA system conducting transmitting and receiving simultaneously. FIG. 15 is a block diagram showing its high-frequency circuit part. This communications terminal comprises a high-frequency SP3T (single-pole, triple-throw) switch 100 connected to one common antenna, a diplexer 200 handling the input/output signals of the CDMA system, transmitting and receiving parts of TDMA, and transmitting and receiving parts of CDMA, the high-frequency switch 100 switching a signal path to the diplexer 200 handling the input/output signals of the CDMA system and to the transmitting and receiving parts of TDMA.
In the high-frequency circuit of JP2000-156651A, the transmitting/receiving signals of CDMA are branched by the diplexer 200. For instance, in the case of W-CDMA, whose transmitting signal and receiving signal have close frequencies, the diplexer 200 should have steep attenuation characteristics. However, because such diplexers are large in size, their use inevitably makes dual-band, mobile phones (multi-band communications apparatuses) large in size. Also, because the TDMA wireless parts and the CDMA wireless parts are switched by the high-frequency switch 100, it is impossible to simultaneously handle the input/output signals of the CDMA and TDMA systems, for instance, to make communications by the TDMA system while receiving the signal of the CDMA system. Of course, communications by the TDMA or CDMA system while receiving a GPS signal are also impossible.
JP2003-8385A discloses composite LC filter circuits (diplexers) for branching signals having three different frequencies. FIG. 16 is a block diagram showing their circuits. These composite LC filter circuits are constituted by a lowpass filter LPF1, one or two bandpass filters BPF, and a highpass filter HPF 1. The lowpass filter LPF 1 connected between the first port P1 and the second port P2 has a passband f1. The highpass filter HPF1 connected between the first port P1 and the second port P4 has a passband f3 (>f1). The bandpass filter BPF connected in series between the first port P1 and the third port P3 has a passband f2 (f1<f2<f3). Exemplified usable systems are AMPS, PCS, GSM, DCS, W-CDMA and GPS. Frequency bands used in the above communications systems are shown in Table 1 below.
TABLE 1CommunicationsTransmitting FrequencyReceiving FrequencySystemBand TX (MHz)Band RX (MHz)GSM880-915925-960DAMPS824-849869-894PDC940-956810-826DCS1710-17851805-1880PCS1850-19101930-1990WCDMA1920-19802110-2170GPS—1574.42-1576.42WLAN2.4-GHz band, 5-GHz band
In the composite LC filter circuits described in JP2003-8385A, each filter is contained in a laminate substrate as an LC filter to achieve size reduction. However, the bandpass filter BPF constituted by an LC filter needs a parallel resonance circuit comprising an inductance element and a capacitance element, another parallel resonance circuit comprising an inductance element and a capacitance element, and pluralities of coupled capacitance elements, inevitably resulting in larger numbers of elements than in the highpass filter and the lowpass filter. Also, when off-band attenuation is not sufficiently achieved, the number of elements for forming resonance circuits, for instance, may have to be increased.
In addition, when the reactance elements are formed by electrode patterns in the laminate substrate, attention should be paid to avoid the deterioration of electric characteristics such as transmission loss and isolation due to floating capacitance, etc. generated by interference between electrode patterns constituting each filter. However, to lay many constituents in the laminate substrate while avoiding interference between patterns, the laminate substrate inevitably has to have a large overall dimension, resulting in difficulty in further size reduction of the high-frequency circuit.